Static electricity
Sparks and sticky balloons
- Rub a balloon on your hair and it sticks to the wall; lightning is the same effect, in giant form.
- Both come from static electricity — charge that has built up and is not moving.
- It all comes down to tiny electrons.
Electric charge
- There are two kinds of charge: positive and negative.
- The rule is like magnetic poles: same charges repel, different charges attract.
- Charge is measured in coulombs (C).
Two negative charges attract each other.
Like charges repel; only different charges (+ and −) attract.
Charging by friction
- Rub two different materials together and electrons (tiny negative particles) move from one to the other.
- The material that gains electrons becomes negative; the one that loses them becomes positive.
- Only the negative electrons move — the positive charge stays put.
- Example: a plastic rod rubbed with a cloth becomes negative (it gained electrons); the cloth is left positive.
When you rub a plastic rod with a cloth, the rod gains electrons. The rod becomes:
Gaining electrons (which are negative) makes the rod negative. The cloth, having lost electrons, is left positive.
During charging by friction, what actually moves between the materials?
Only the negative electrons move. The positive charge stays fixed in place.
Conductors and insulators
- A conductor (copper and other metals) lets charge flow easily, because it has free electrons.
- An insulator (plastic, rubber, glass) does not — its electrons cannot move freely.
- Test it: put the material in a circuit with a lamp and a battery — the lamp lights only for a conductor.
Why is copper a good conductor of electricity?
A conductor has free electrons that can flow through it. Insulators have no free-moving charges.
Electric fields and ions
- An electric field is a region where a charge feels a force.
- Its direction at a point is the direction of the force on a positive charge.
- Field patterns: lines point away from + and toward −; between parallel plates the lines are straight and evenly spaced.
- An atom that loses electrons becomes a positive ion; one that gains electrons becomes a negative ion.
The direction of an electric field at a point is the direction of the force on:
By convention, the electric field direction is the direction of the force on a positive charge.
A neutral atom loses two electrons. It becomes:
Losing negative electrons leaves more positive charge, so the atom becomes a positive ion.
You've got it
- two charges: + and −; same repel, different attract
- charging by friction moves electrons only: gain → negative, lose → positive
- conductors have free electrons; insulators do not
- electric field direction = force on a positive charge; lose electrons → positive ion